In recent years, in the field of material processing, there have been demands for laser devices having an output power of kW. It is difficult, however, to achieve such a laser device with the use of a single fiber laser. Under the circumstances, the use of a fiber laser system, which includes a plurality of fiber lasers and a combiner for combining laser beams outputted from the respective fiber lasers, has been started in the field of material processing.
With such a fiber laser system, there are cases where a laser beam reflected from an object which is being processed (hereinafter referred to as “processing object”) re-enters the fiber laser system, and the fiber laser system consequently encounters a problem.
For example, if a backward laser beam, which is propagating in a backward direction in a fiber laser system after being reflected by a processing object, is added to a forward laser beam which is propagating in a forward direction in the fiber laser system, then a power of a laser beam incident on each point in the fiber laser system becomes remarkably high. In such a case, stimulated Raman scattering at each point in the fiber laser system is accelerated, so that oscillation of Stokes beam may occur. It is known that if oscillation of Stokes beam occurs in a fiber laser system, then (i) laser oscillation in fiber lasers becomes unstable and/or (ii) the fiber lasers malfunctions (see Patent Literature 1).